(a) Field of the Invention
The present invention relates to an objective for an endoscope.
(b) Description of the Prior Art
A retrofocus type lens system disclosed in U.S. Pat. No. 4,037,938 and having a lens configuration as shown in FIG. 1 is known as an objective for an endoscope. Said known objective comprises a front subsystem F having negative refractive power, and a rear subsystem R having positive refractive power, and the lens system as a whole is arranged as a telecentric type lens system by providing a stop S at the front focal point of the rear subsystem. Said known objective has a strong point that the loss of light to be caused in the image guide fiber bundle 1 is small because the prinicpal ray 2 vertically enters the light entrance surface of the image guide fiber bundle 1. However, as said objective is composed in such lens type that is considerably asymmetrical in respect to the stop S, distortion occurs largely. Moreover, as said objective is arranged as a telecentric type lens system, the outer diameter of the lens system is large compared with the image height.
On the other hand, a symmetrical type lens system disclosed in U.S. Pat. No. 2,781,695 and having a lens configuration as shown in FIG. 3 is known as a lens system whose distortion is very small. Said known lens system is arranged to be perfectly symmetrical in respect to the stop S as it will be understood also from FIG. 3 and, therefore, distortion to be caused is very small. However, due to the fact that the lens system is symmetrical, the incident angle on the first lens component L.sub.1 is equal to the emerging angle from the fourth lens component L.sub.6 and, therefore, the incident angle of the principal ray with a large angle of view which is incident on the image surface becomes considerably large. As a result, the intensity of light in the marginal portion decreases due to the cos.sup.4 law.
Furthermore, modified symmetrical type lens systems are also known as disclosed in U.S. Pat. No. 3,209,646 and U.S. Pat. No. 4,176,915. Said known lens systems have the basic power distribution similar to that of the lens system shown in FIG. 3, i.e., a negative and positive lens components are arranged in front of the stop and a positive and negative lens components are arranged in rear of the stop. Besides, shapes of individual lens components arranged in front of the stop are substantially the same as those of individual lens components arranged in rear of the stop.